The present invention relates generally to a suspension control system for an automotive vehicle with variable damping force depending upon road surface condition. More specifically, the invention relates to a suspension control system which includes a sensor monitoring road surface conditions for use in controlling the stiffness of the suspension in accordance therewith.
Various uses of road preview sensors have been proposed and developed. For example, SAE Technical Paper Series Nos. 680750 and 800520, respectively published on Oct., 1968 and Feb., 1980 show road preview sensors for use in suspension systems for obtaining optimum riding comfort and drivability. In addition, Japanese Patent First Publication No. 57-172808, published on Oct. 23, 1982 discloses a vehicle height control system which includes a sensor which detects rough road conditions and adjusts the vehicle height level depending upon road surface conditions. A vehicle height or level sensor is employed in the disclosed vehicle height control system for monitoring the relative displacement between the vehicle body and wheel axle. The output of the vehicle level sensor is compared with a reference level, which serves as a rough road criterion, and adjusts the vehicle height according to the result of judgement of the road surface conditions.
In another example, Japanese Patent First Publication No. 58-30542, published on Feb. 23, 1983, discloses a variable damping force shock absorber with damping characteristics varying in accordance with vehicle driving conditions. In the disclosed system, the magnitude of relative displacement between the vehicle body and wheel axle is measured and a vehicle height variation indicative signal is derived from the measured displacement and the instantaneous vehicle speed. The vehicle height variation indicative signal value is compared with a reference value which serves as a staff suspension criterion for adjustment of the damping characteristics of the shock absorber in accordance therewith.
Conventional suspension control systems encounter difficulty in recognizing the nature of vibrations causing relative displacement between the vehicle body and the wheel axle. For instance, when the road wheel vibrates due to small-scale irregularities in the road surface, a softer or weaker suspension may be preferred in order to provide sufficient riding comfort. On the other hand, when the vehicle body vibrates on a larger scale, i.e. if it starts to roll or pitch, a stiffer suspension is preferable to provide riding comfort and better drivability.